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Ionospheric Tomography

Tomography is a mathematical technique to reconstruct two-dimensional images and it is best known for the reconstruction of the human body from the X-ray measurements. First attempts of imaging the Earth’s ionosphere with tomography have been made in the mid ’80, to determine the distribution of free electron concentration from satellite-to-ground radio signals. Nevertheless, only during the 90’s, the availability of the GPS constellation has allowed to achieve satisfied results, providing information on the total electron content (TEC) through the measurements of phase delay and pseudorange of the signals (Fig. 1).

Figure 1 Scheme of the GPS satellites geometry showing how their signals can provide information on the TEC along the satellite-receiver path

The GPS satellites are monitored by a network of dual-frequency receivers (L1:1,5 GHz, L2:1,1 GHz), able to record phase and time delay from any available signal. The international IGS (International GPS Service) network is spread all over the world and it is open for data downloading providing also a valuable source of information about the ionospheric delay. Nevertheless, these measurements are rather uneven in distribution and coverage, leaving uncovered wide areas as the oceans or Antarctica. Another critical drawback is that tomographic images are only two dimensional (latitude vs. altitude slices), whereas full three-dimensional images are usually needed. INGV is cooperating with the University of Bath since several years to use and further develop a tomographic technique called MIDAS (Multi-Instrument Data Analysis System). MIDAS is unique in its approach to ionospheric imaging, because it performs a four-dimensional (i.e. 3D time-dependent) inversion, offering the possibility follow the temporal evolution of the ionosphere till timescales of minutes. The MIDAS technique has been successfully applied to realize the MIRTO (Mediterranean Ionosphere with Real-time TOmography) prototype, a collaborative project between Istituto Nazionale di Geofisica (INGV) of Rome, the University of Bath (UK) and the Istituto Fisica Applicata “Nello Carrara”- Consiglio Nazionale delle Ricerche (IFAC-CNR) of Florence, funded by MIUR. The MIRTO project sets the aim of the realization of a prototype that produces images over Italy, with extension to the Mediterranean area, in near real-time. The crucial point of any inversion problem is to reduce an equations system admitting infinite solutions to a finite number. To do that it is necessary to adopt proper mathematical techniques and to ingest all the available information on the medium to be reconstructed. MIDAS reduces the inversion problem to a finite number of solutions through the adoption of the orthogonal functions system, known as “Empirical Orthogonal Functions” (EOF) and it assimilates different types of data: plasma frequencies and virtual heights of the ionospheric layers (provided by the vertical soundings), the GPS measurements, the LEO (Low Earth Orbit) measurements, etc. This wide flexibility is crucial in zones well covered, as the Mediterranean area over which INGV operates with several kind instruments and over which the IGS network is particularly rich, and in regions scarcely covered as Antarctica, for which, vice versa, the possibility to use any kind of available information is fundamental to solve the problem.

MIRTO provides real-time maps and movies of the electron density on the Mediterranean area (Fig.2), providing information useful for scientific purposes as well as for technological applications. MIRTO contributes to the knowledge of the ionospheric plasma under quiet and disturbed magnetic conditions, furnishing useful indications for the realization of forecasting models addressed to the space weather. Moreover, MIRTO is a valid tool of electromagnetic monitoring of the Mediterranean area useful also to verify the existence of ionospheric events due to seismic occurrences.

Figure 2 TEC map produced by MIRTO for February 22, 2006 at 13:01 UT. The colour bar indicates the TEC units (TECU= 1016 electrons/m2). The map has been realized using the GPS signals of all the satellites in view during the considered period (red tracks on the left)

MIDAS has been recently applied to produce, for the first time, a tomographic imaging of the ionosphere over Antarctica (Fig.3). This first attempt arises from a collaboration between the geodetic and upper atmosphere physics communities managing GPS receivers in Antarctica and it has been recently approved and funded by SCAR (Scientific Committee for Antarctic Research), in the frame of the UAMPY (Upper Atmosphere Monitoring for the international Polar Year) project.

Figure 3 TEC reconstruction over the southern hemisphere produce by MIDAS

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